My internet life is reduced to ruins, my music listening life just manages to survive, and yesterday, so as to relax and forget about a blistering headache, I decided to try and play the piano for the first time in over a year (noisy as that may be, it works miracles with my -luckily occasional- headaches).

I picked the first piece of sheet music I found, and while I was playing I realised something I hadn't noticed previously. It's a weird notation I'd never seen before. I went to my collection of old sheet music and found more exampes of it (so I'd actually seen it before, or rather, I'd had it before my eyes before without being conscious of it). I cannot figure out what it is, but since it only appears in British scores, I thought someone here will probably know what on earth this notation is, mainly music-oriented people like you. Or maybe non-British people will know and it's only me who is not analytical or wise enough.

The scores are the typical arrangement of pop songs (late 50s - early 60s) for voice and piano, with chord names too. But then, above the voice staff there is this weird series of letters, dots and lines, like this:

d' :t |l :s m : .m l : _ m :_.m

And so on. I've only been able to figure out that each letter corresponds to a note, and that in this particular score l is always C, t is always D, d is always E... (In other scores each letter matches a different note).

Can you explain me what on earth this is or, at least, what instrument it's meant for?